Optical measurement for measuring a small space through a transparent surface

ABSTRACT

The invention provides a system and method for reliably and accurately measuring the gap between two materials when the depth of gap is less than the smallest distance that an optical thickness gauge (OTG) is able to measure. The invention is practiced by forming a suitable slot (or a groove, channel, hole or other suitable deformation) having a precisely known depth in at least one material. The sum of the distance of the gap and the depth of the slot is at least equal to the smallest distance that the OTG can measure. The slot is positioned over the materials and under the OTG probe head such that a cavity is formed. The depth of the cavity is measured. Since the distance of the slot is known, the depth of the gap is determined by subtracting the known depth of the slot from the measured depth of the cavity.

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to optical reflectometry,and more particularly, to a system and method for measuring a gapbetween two surfaces. With the advent of optical reflectometry-basedmeasuring devices capable of distances as small as 10 microns (μm),precise and accurate measurements of critically small distances can bemade. A nonlimiting example of an optical reflectometry-based measuringdevice is the optical thickness gauge (OTG) once sold by Hewlett-Packard(HP 86125A-K1X). The operation and functionality of such an OTG isdisclosed in of U.S. Pat. Ser. No. 5,642,196, filed on Jun. 24, 1997,and entitled METHOD AND APPARATUS FOR MEASURING THE THICKNESS OF A FILMUSING LOW COHERENCE REFLECTOMETRY, which is entirely incorporated hereinby reference. Other exemplary optical reflectometry-based measuringdevices and their applications, incorporated herein by reference, aredisclosed in U.S. Pat. Ser. No. 5,473,432, filed on Dec. 5, 1995, andentitled APPARATUS FOR MEASURING THE THICKNESS OF A MOVING FILMUTILIZING AN ADJUSTABLE NUMERICAL APERTURE LENS, U.S. Pat. Ser. No.5,610,716, filed on Mar. 11, 1997, and entitled METHOD AND APPARATUS FORMEASURING FILM THICKNESS UTILIZING THE SLOPE OF THE PHASE OF THE FOURIERTRANSFORM OF AN AUTOCORRELATOR SIGNAL, U.S. Pat. Ser. No. 5,633,712,filed on May 27, 1997, and entitled METHOD AND APPARATUS FOR DETERMININGTHE THICKNESS AND INDEX OF REFRACTION OF A FILM USING LOW COHERENCEREFLECTOMETRY AND A REFERENCE SURFACES, U.S. Pat. Ser. No. 5,646,734,filed on Jul. 8, 1997, and entitled METHOD AND APPARATUS FORINDEPENDENTLY MEASURING THE THICKNESS AND INDEX OF REFRACTION OF FILMSUSING LOW COHERENCE REFLECTOMETRY, U.S. Pat. Ser. No. 5,642,196, filedon Jun. 24, 1997, and entitled METHOD AND APPARATUS FOR MEASURING THETHICKNESS OF A FILM USING LOW COHERENCE REFLECTOMETRY, U.S. Pat. Ser.No. 5,731,876, filed on Mar. 24, 1998, and entitled METHOD AND APPARATUSFOR ON-LINE DETERMINATION OF THE THICKNESS OF A MULTILAYER FILM USING APARTIALLY REFLECTING ROLLER AND LOW COHERENCE REFLECTOMETRY, and U.S.Pat. Ser. No. 5,850,287, filed on Dec. 15, 1998, and entitled ROLLERASSEMBLY HAVING PRE-ALIGNED FOR ON-LINE THICKNESS MEASUREMENTS.

[0002] A conventional optical thickness gauge (OTG) is used to measuredsmall distances between surfaces, such as a gap or separation betweentwo materials. However, the OTG is limited in that there is somedistance that is the smallest distance that the OTG can measure. Thatis, distances smaller than the smallest distance that the OTG canmeasure are less than the TG resolution capability, and therefore cannot be determined. For example, one conventional type of OTG has aresolution of 10 microns (μm). Distances less than 10 μm can not bedetermined with a sufficient degree of accuracy and/or reliability.

[0003]FIG. 1 is a block diagram illustrating a conventional OTG 100using a prior art method of measuring distances associated with amulti-layer film 102 and in communication with a personal computer (PC)104. The OTG 100 has at least a low-coherence light source 106, anoptical coupler 108, an autocorrelator 110 and a probe head 112.Low-coherence light 114 is generated by the low-coherence light source106 and injected into waveguide 116. Waveguide 116 may be any suitabledevice, such as an optical fiber, that is configured to transfer thelow-coherence light 114 to the optical coupler 108. The low coherencelight 114 propagates through the optical coupler 108, through thewaveguide 118 and into the probe head 112. Light is reflected back intothe probe head 112, in a manner described below, through the waveguide118, through the optical coupler 108, through the waveguide 120. Thereturn light 122 is detected by the autocorrelator 110 so that distancemeasurements can be determined, as described below, by software (notshown) residing in PC 104.

[0004] For convenience of illustration, the waveguide 116 is illustratedas having a separation distance from the low-coherence light source 106.One skilled in the art will appreciate that the waveguide 116 would betypically coupled directly to the low-coherence light source 106 usingwell known coupling devices. Likewise, the waveguide 120 is illustratedas having some amount of separation from the autocorrelator 110.Waveguide 120 is typically coupled directly to the autocorrelator 110.For convenience of illustration, the waveguide 118 is illustrated asbeing directly coupled to the optical coupler 108 and probe head 112.Coupling devices used to couple the waveguides 116, 118 and 120 todevices are well known in the art and are not described in detail orillustrated herein. Furthermore, for convenience of illustration, thewaveguides 116, 118 and 120 are illustrated as a rod-like materialintended to represent a flexible optical fiber. However, any suitablewaveguide device configured to transmit light between the low-coherencelight source 106, the optical coupler 108, the autocorrelator 110 andthe probe head 112, may be substituted for the waveguides 116, 118 and120.

[0005] The optical autocorrelator 110 is configured to receive thereturn light 122. Detectors (not shown) residing in the autocorrelator110 generates information such that the autocorrelator 110 generatescorrelation peaks that are shown on the graph 128. Separation betweencorrelation peaks corresponds to distances between any two lightreflecting surfaces.

[0006] Optical correlator 110 is coupled to the PC 104 via theconnection 124. Information from the autocorrelator 110 is received bythe PC 104 and processed by software (not shown) into correlationinformation. The PC 104 typically displays, on the display screen 126,the correlation results as a graph 128 having correlation peaks,described in greater detail below. That is, distances betweencorrelation peaks correspond to the measurements taken by the OTG 100.

[0007] For convenience of illustration, the PC 104 is illustrated as aconventional laptop PC. However, any suitable PC or other processingdevice may be equally employed for the processing of informationcorresponding to the light signals received by the autocorrelator 110,and to prepare a meaningful output format that is interpreted by a userof the OTG 100 for the determination of distances. Furthermore, thedisplay 126 may be any suitable device for indicating distanceinformation resulting from measurements taken by the OTG 100. Forexample, but not limited to, the display 126 may be a conventional,stand-alone cathode ray tube (CRT). Or, a line printer, plotter, orother hard copy device may be configured to accept and indicatecorrelation information from the autocorrelator 110.

[0008] Light (not shown), entering the probe head 112 via the waveguide118, first passes through a reference surface 130. Here, the referencesurface 130 is illustrated as the bottom surface of a wedge-shaped plate131. (For convenience of illustration, wedge-shaped plate 131 is shownfrom an edge-on viewpoint.) Reference surface 130 is configured to allowa portion of the received light to pass through the wedge-shaped plate131 and onto the film 102. A portion of the received light (not shown)entering the wedge-shaped plate 131 is reflected from the referencesurface 130, back through the probe head 112, through the waveguide 118,through the optical coupler 108 and then through the waveguide 120 to bereceived by the autocorrelator 110.

[0009]FIG. 2 is a simplified graph 200 illustrating the correlationpeaks associated with the reflection of light from the reference surface130 and the surfaces 132, 134, 136 and 138 of film 102 (FIG. 1) usingthe prior art method of measuring distances. For convenience ofillustrating the autocorrelation information on the graph 200, thevertical axis corresponding to the magnitude of the correlation peaks isnot numbered. One skilled in the art will realize that any appropriatevertical axis numbering system corresponding to the amplitude of thecorrelation peaks could have been employed, and that such a numberingsystem is not necessary to explain the nature of the correlation peaks.Similarly, the horizontal axis corresponding to distance has not beennumbered on the graph 200. One skilled in the art will realize that anyappropriate axis number system corresponding to distance could have beenemployed, and that such a numbering system is not necessary to explainthe nature of the relationship between the correlation peaks illustratedin the graph 200. Thus, one embodiment of the software generating thegraph 200 is configured to allow the user of the PC 104 to alter thehorizontal and the vertical axis numbering systems so that the locationof the correlation peaks of interest, and their relative separationcorresponding to distance, can be meaningfully discerned and determinedby the user of the PC 104 (FIG. 1).

[0010] Information received from the autocorrelator 110 is processed bythe PC 104 (FIG. 1) such that the correlation peak 202 is plotted at thereference point (x=0) on the graph 200. Correlation peak 202 is a largepeak, plotted at the zero or reference point on the x-axis of the graph200, that corresponds to the correlation of each the reflected lightportions with itself.

[0011] Returning to FIG. 1, the portion of light passing through thereference surface 130, referred to as the incident beam 140, passesthrough air for a suitable distance before striking the first surface132 of film 102. When the incident beam 140 shines upon the surface 132,a portion of the incident beam 140 is reflected from the surface 132, asa reflected light beam 142, back up through the probe head 112, throughthe waveguide 118, through the optical coupler 108, through thewaveguide 120, and then is received by the autocorrelator 110. Theautocorrelator 110, based upon the time delay between the lightreflected from the reference surface 130 and the reflected light beam142, determines a correlation peak 204 (FIG. 2) as illustrated on thegraph 200. Typically, the magnitude of the reflected light beam 142 isrelatively small. Thus, the correlation peak 204 is significantly lessin magnitude than the correlation peak 202, as illustrated in the graph200. The user of the PC 104 viewing the graph 200 interprets therelative separation between the correlation peaks 202 and 204 ascorresponding to a distance 144 between the referenced surface 130 andthe surface 132 of the film 102.

[0012] For convenience of illustration, the incident beam 140 and thereflected light beams 142, 154, 158 and 162 reflected from surfaces 132,134, 136 and 138, respectively, are shown at slight angles. However, oneskilled in the art will appreciate that the incident beam 140 and thelight beams 142, 154, 158 and 162 are all orthogonal to the referencesurface 130 and the surfaces 132, 134, 136 and 138. Furthermore, forconvenience of illustration, because the distance 144 is typically muchgreater than the distances of interest associated with the film 102,only a portion of the distance between the correlation peaks 202 and 204is illustrated. Thus, a portion of the horizontal axis and a portion ofthe distance between the correlation peaks 202 and 204 is omitted fromthe graph 200, as indicated by the break line 206.

[0013] One skilled in the art will appreciate that the separationbetween the correlation peaks 202 and 204 is function of a variety ofwell known physical factors. Light travels at a finite speed. The speedof the light is affected by the medium through which the light istraveling. Thus, one skilled in the art will readily appreciate that twosignificant factors in determining the time delay of the variousportions of light detected by the autocorrelator 110 are the totaldistance traveled by the light, and the properties of the various mediumthrough which the light travels. For example, the reflected light beam142 travels from the reference surface 130 to the surface 132, and thenreturns back to the reference surface 130. Therefore, because thereflected light beam 142 travels farther than the light reflecting fromthe reference surface 130, and because the light beam 142 travelsthrough air, the light beam 142 requires more time to reach theautocorrelator 110 than the time required by the light reflecting fromthe reference surface 130. The physical properties associated with themediums through which the light travels is defined by the well knownrefractive index (n) of the material. Thus, software analyzing therelative separation between the correlation peak 202 and the correlationpeak 204 accurately calculates the distance 144 and provides thatinformation to the user of the PC 104. This information is communicatedby appropriately labeling the horizontal axis of FIG. 2, and/orproviding a numerical figure to the user. Such a process of determiningdistances with an OTG 100 (FIG. 1) is well known in the art and is notdescribed in further detail herein.

[0014]FIG. 1 illustrates the OTG 100 measuring distances associated withfilm 102. For convenience of illustration, the film 102 has threelayers; a top layer 146, a middle layer 148 and a bottom layer 150. Thelayers 146, 148 and 150 are made from different materials bondedtogether to create a single layer of film 102. Typically, the film 102is a long, continuous roll or sheet of flexible material. However, forconvenience, only a portion of the roll or sheet of the film 102 isshown in FIG. 1, as illustrated by the cut-away lines 152. Furthermore,the layers 146, 148 and 150 must be sufficiently transparent so theincident beam 140 travels through, and light reflected back through thelayers 146, 148 and 150.

[0015] Each layer 146, 148 and 150 have different refractive index (n).Surface 132 corresponds to the transition between air and the film 102,and thus corresponds to a change in the refractive index of air to therefractive index of the top layer 146. Similarly, surface 134corresponds to the transition between the material of top layer 146 andthe material of middle layer 148. Surface 136 corresponds to thetransition between the middle layer 148 and the bottom layer 150.Surface 138 corresponds to the bottom surface of film 102, and alsocorresponds to a transition between the bottom layer 150 and thematerial that the film 102 is residing in, such as air. Each of thesesurfaces are also characterized by a change in refractive index.

[0016] When the incident beam 140 is incident on the surface 134, aportion of the incident beam 140 passes through the surface and aportion of the incident beam 140 is reflected back up to the probe head112 because of the difference in the refractive index n of the layers146 and 148. The amount of reflected light corresponds, in part, to thedegree of difference between the refractive index n. Thus, when theincident beam 140 passes through the top layer 146 into the middle layer148, the reflected light beam 154 is reflected from the surface 134 backup through the top layer 146 and into the probe head 112. The reflectedlight beam 154 is eventually detected by the autocorrelator 110 in themanner described above. Because of the time delay between the reflectedlight beam 154 from the surface 134 with respect to the light reflectedfrom the reference surface 130, a correlation peak 208 (FIG. 2) will bedetermined. Furthermore, since the time delay between the reflectedlight beam 154 from the surface 134, with respect to the reflected lightbeam 142 from the surface 132, is equal to the time required for thelight to travel through the layer 146 only, the separation betweencorrelation peak 204 and correlation peak 208 is proportional to thedistance 156 and the index of refraction of the layer 146.

[0017] Similarly, a portion of the incident beam 140 incident on thesurface 136 corresponding to the material transition between the middlelayer 148 and the bottom layer 150, is reflected back up to the probehead 112 as reflected light beam 158. Because of the time delayassociated with the reflected light beam 158 with respect to the lightreflected from the reference surface 130, a correlation peak 210 (FIG.2) is determined. Furthermore, since the time delay between thereflected light beam 158 from the surface 136, with respect to thereflected light beam 154 from the surface 134, is equal to the timerequired for light to travel through the layer 148 only, the separationbetween the correlation peak 208 and the correlation peak 210 isproportional to the distance 160 in the index of refraction of the layer148.

[0018] Likewise, a portion of the incident beam 140 will be reflectedfrom the surface 138 back up to the probe head 112 as a reflected lightbeam 162. Because of the time delay associated with the reflected lightbeam 162 with respect to the light reflected from the reference surface130, a correlation peak 212 (FIG. 2) is determined. Furthermore, sincethe time delay between the reflected light beam 162 from the surface138, with respect to the reflected light beam 158 from the surface 136,is equal to the time required for light to travel through layer 150only, the separation between the correlation peak 210 and thecorrelation peak 212 is proportional to the distance 164 and the indexof refraction of the layer 150. In some applications, the bottom surface138 of the film 102 is coated with a highly reflective surface to causea large portion of the incident beam 140, or all of the remainingincident beam 140, is reflected up to the probe head 112 as thereflected light beam 162. Thus, the correlation peak 212 is illustratedas having a relatively greater magnitude than the correlation peaks 204,208 and 210 (FIG. 2).

[0019] The autocorrelator 110 (FIG. 1) generates a correlation peak forall pairs of reflections from any two surfaces. However, for convenienceof illustrating the graph 200 (FIG. 2), not all correlation peaks areillustrated. When spatial separation between the film surfaces 132, 134,136 and 138 (FIG. 1) are sufficient, correlation peaks generated by thecorrelation of the reference surface 130 with each of the film surfaces132, 134, 136 and 138 are used to make measurements of the thickness ofthe film layers 146, 148 and 150 (FIG. 1). Alternatively, the topsurface 132 may be used instead of reference surface 130 to determinecorrelation peaks.

[0020] One skilled in the art will appreciate that many correlationpeaks will be displayed on the graph 200, and that one skilled in theart will employ experience in using the OTG 100 (FIG. 1) to determinewhich correlation peaks are relevant to the particular measurements ofinterest. Thus, for convenience of illustration, the correlation peaksillustrated on the graph 200 are limited to correlation peaks that areconvenient in explaining the operation and functionality of the OTG 100.

[0021] Summarizing, the OTG 100 shines a low-coherence incident beam 140onto the film 102 so that portions of the incident beam 140 arereflected back to the OTG 100 (reflected light beams 142, 154, 158 and162) and detected by the autocorrelator 110. Software analyzes the timedelays associated with the reflected light beams 142, 154, 158 and 162,with respect to the light reflected from reference surface 130, todetermine the distances 144, 156, 160 and 164, respectively. The abilityto resolve the minimum peak separation is determined by thecoherence-length of the light source. Thus, a lower-coherence lengthlight source gives a higher resolution. One commercially available OTG100 is capable of discerning distances as small as 10 μm.

[0022] However, the above-described commercially available OTG 100 isnot capable of measuring with any degree of reliability and accuracy ofdistances smaller than 10 μm. Even as technologies advance such that theresolution of more advanced OTGs provide for measuring distances smallerthan 10 μm, there will always be some minimum distance that an OTG isable to measure within an acceptable degree of reliability and accuracy.Distances less that this minimum distance can not be measured with anacceptable degree of reliability and accuracy. Thus, a heretoforeunaddressed need exists in the industry for providing a system andmethod of accurately and reliably measuring distances that are smallerthan the minimum distance that an OTG can reliably and accuratelymeasure.

SUMMARY OF THE INVENTION

[0023] The present invention reliably and accurately measures a gapbetween two materials when the depth of the gap is less than thesmallest distance that the measuring device, such as an opticalthickness gauge (OTG), is able to reliably and accurately measure. Forexample, if an OTG is capable of measuring distances as small as 10microns (μm), the invention allows accurate and reliable measurement ofa gap having a distance that is smaller than 10 μm. The invention ispracticed by forming a suitable recess in at least one of the materials.Examples of such a recess include a slot, groove, channel, hole or othersuitable deformation. The depth of the recess is precisely known. Thus,the sum of the distance of the gap and the depth of the recess is atleast equal to the smallest distance that the OTG can measure with anacceptable degree of reliability and accuracy. The recess may be formedin either material. In an alternative embodiment, the recess is formedin both materials.

[0024] The recess is positioned over the materials and under the probehead of the OTG to form a measurable region or cavity. The depth of thecavity is precisely measured. Since the distance of the recess isprecisely known, and the depth of the cavity is measurable, the depth ofthe gap is easily determined by subtracting the known distance of therecess from the measured depth of the cavity. Thus, the inclusion of therecess in at least one of the materials enables the OTG to accuratelyand reliably determine the depth of the gap. Hereinafter, the term“slot” is used interchangeably with the term “recess” for convenience.

[0025] In another embodiment, the depth of the slot is not preciselyknown when the slot is formed in the material. However, the depth of theslot is at least equal to the distance that the OTG can reliably andaccurately measure. Thus, the depth of the slot is determinable bymeasurement.

[0026] The present invention can also be viewed as providing a methodfor measuring distance between two materials. The method includes thesteps of measuring a distance between a slot surface formed by a slot ina first material and a surface on a second material (the first materialhaving a precisely known distance between the slot surface and thesurface of the first material); and subtracting from the measureddistance the precisely known distance to determine the distance betweenthe first material and the second material.

[0027] Other features and advantages of the present invention willbecome apparent to one skilled in the art upon examination of thefollowing detailed description, when read in conjunction with theaccompanying drawings. It is intended that all such features andadvantages be included herein within the scope of the present inventionand protected by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The invention can be better understood with reference to thefollowing drawings. The elements of the drawings are not necessarily toscale relative to each other, emphasis instead being placed upon clearlyillustrating the principles of the present invention. Furthermore, likereference numerals designate corresponding parts throughout the severalviews.

[0029]FIG. 1 is a block diagram illustrating a conventional opticalthickness gauge (OTG) using a prior art method of measuring distancesassociated with a film and in communication with a PC.

[0030]FIG. 2 is a simplified graph illustrating the correlation peaksassociated with the reflection of light from the reference surface andthe surfaces of the film layers of FIG. 1 using the prior art method ofmeasuring distances.

[0031]FIG. 3 is a block diagram illustrating the OTG of FIG. 1 measuringa gap between two materials.

[0032]FIG. 4 is a simplified graph illustrating the correlation peaksassociated with the reflection of light from the surfaces of thematerials of FIG. 3.

[0033]FIG. 5 is a block diagram illustrating the OTG of FIG. 1 measuringa gap between two materials having a slot disposed in the bottommaterial of FIG. 3.

[0034]FIG. 6 is a block diagram illustrating the OTG of FIG. 1 measuringa gap between the two materials of FIG. 3, each having a slot disposedin the materials.

[0035]FIG. 7 is a block diagram illustrating the OTG of FIG. 1 measuringa gap between the two materials of FIG. 3.

DETAILED DESCRIPTION

[0036]FIG. 3 is a block diagram illustrating an optical thickness gauge(OTG) 100 measuring a gap 302 between two materials 304 and 306. The gap302 has a distance 308 that is less than the smallest resolutiondistance of the OTG 100. For example, if the OTG 100 is capable ofmeasuring distances as small as 10 microns (μm), the present inventionillustrated in FIG. 3 accurately and reliably measures the gap 302having a distance 308 that is smaller than 10 μm.

[0037] The operation of the OTG 100 is described above. Therefore, theoperation and functionality of the OTG 100 is not described again indetail other than to the extent necessary to understand how the distance308 of the gap 302 is reliably and accurately determined.

[0038] The present invention is practiced by disposing a recess,hereinafter referred to as slot 310, in the material 304. Slot 310 has adepth in which the distance 312 is precisely known. For example, thedepth of the slot 310 may be determined by measurement using the OTG100, by a higher resolution OTG, or by other conventional devicescapable of measuring small distances directly or indirectly. Slot 310 isdisposed into the material 304 in any suitable manner so long as thedepth of the slot 310, (distance 312) is precisely known. Slot 310 formsa surface 330 in material 304. Surface 330 is substantially uniform orflat and does not have any significant surface irregularities ordistortions. For example, but not limited to, the slot 310 may becarved, cut or etched into the material 304. In another embodiment, theslot 310 may be included as part of a mold used during the fabricationof the material 304. Slot 310 may be fabricated into the material 304using any suitable method and/or means to form a precisely known depth(distance 312) of the slot 310. All such methods and/or means forfabricating the slot 310 in the material 304 and for measuring the depthof the slot 310 are intended to be included herein within the scope ofthis disclosure.

[0039] Material 304 is substantially transparent. Thus, the incidentbeam 314 travels through the material 304. For convenience ofillustration, only a portion of the material 304 is shown, as indicatedby the cut-away lines 316. Below the material 304 is a material 306. Forconvenience of illustration, only a portion of the material 306 isshown, as indicated by the cut-away lines 318.

[0040] The material 304 and the material 306 are separated by a gap 302.The distance 308 corresponds to the depth of the gap 302. In situationswhere the distance 308 is smaller than the minimum distance that the OTG100 is capable of measuring, the slot 310 is positioned over thematerial 306 and under the probe head 112 to form a measurable region(cavity 320). The cavity 320 is associated with the distance 322. Thedistance 322 is at least equal to the minimum distance that the OTG 100is capable of measuring. The distance 322 corresponds to the sum of thedistances 308 and 312. Since the distance 312 is precisely known, andthe distance 322 is measurable, the distance 308 is easily determined bysubtracting the known distance 312 from the measured distance 322. Thus,the inclusion of the slot 310 in the material 304 enables the OTG 100 toaccurately and reliably measure the distance 308 of the gap 302.

[0041] When an incident beam 314 is incident upon the top surface 324 ofthe material 304 a portion of the incident beam 314 is reflected fromsurface 324. Thus, a light beam 326 is reflected back up into the probehead 112. The reflected light beam 326 is detected by the autocorrelator110. The difference in the time delay of the reflected beam 326,relative to the time delay of a light beam (not shown) reflected fromthe reference surface 130 results in a correlation peak 402 (FIG. 4)that is displayed on the graph 400 (FIG. 4).

[0042] The unreflected portion of the incident beam 314 travels throughthe transparent, or partially transparent, material 304 and is incidentupon the top surface 330 of the slot 310 such that a reflected lightbeam 332 is reflected back up through the transparent material 304 intothe probe head 112. The reflected light beam 332 is detected by theautocorrelator 110. The difference in the time delay of the reflectedlight beam 332, relative to the time delay of the reflected light beam(not shown) from reference surface 130 results in a correlation peak 404(FIG. 4) that is displayed on the graph 400 (FIG. 4).

[0043] The unreflected portion of the incident beam 314 continuestraveling through the transparent material 304, through the cavity 320,and is incident upon the top surface 334 of the material 306. A lightbeam 336 is reflected off of the surface 334 up through the gap 302, upthrough the cavity 320, up through the transparent material 304, andthen back up into the probe head 112. The reflected light beam 336 isdetected by the autocorrelator 110. The difference in the time delay ofthe reflected light beam 336, relative to the time delay of thereflected light beam (not shown) from reference surface 130, results ina correlation peak 406 (FIG. 4) displayed on the graph 400 (FIG. 4).

[0044] Depending upon the width of the slot 310, the spot size of theincident beam 314, and the positioning of the probe head 112 over theslot 310, a portion of the incident beam 314 may be incident on thebottom surface 338 of the material 304. If so, then a portion of theincident beam 314 will be reflected back up into the probe head 112 asreflected light (not shown for convenience of illustration since thisreflected light is not material to determining the distance 308 of gap302 in this embodiment). This reflected light from the surface 338 willbe detected by the autocorrelator 110. Thus, a correlation peak 408 isdisplayed on the graph 400 (FIG. 4).

[0045] If the material 306 is opaque, or if the material 306 includes ahighly reflective surface coating (not shown) on the surface 334, thenthe incident beam 314 will not pass into the material 306. In such acase, there will not be reflections of light from other surfaces backinto the probe head 112. In such a situation, the correlation peak 406is typically much higher than the correlation peaks 402 and 406 becausethe reflected light beam 336 is stronger (greater strength) than thereflected light beams 326 and 332.

[0046] Alternatively, if the material 306 is transparent, or partiallytransparent, there will be other reflections of light (not shown) backinto the probe head 112. However, such other reflections are notnecessarily relevant to the operation of the OTG 100 in determining thedistance 308 of the gap 302, and therefore, are not discussed againherein, nor illustrated in the several figures. In such a situation, anyresultant correlation peaks (not shown) are expected to be discernablefrom the correlation peaks 402, 404 and/or 406 illustrated on the graph400.

[0047] For convenience of illustrating light reflected from the surfaces324, 330 and 334, the incident beam 314 and reflected light beams 326,332 and 336 are shown at slight angles. However, one skilled in the artwill appreciate that the incident beam 314 and the reflected light beams326, 332 and 336 are all orthogonal to the surfaces 324, 330 and 334.

[0048] For convenience of illustrating graph 400 (FIG. 4), not allcorrelation peaks are illustrated. Autocorrelator 110 (FIG. 3) generatesa correlation peak for all pairs of reflections from with any twosurfaces. For example, autocorrelator would determine a correlation peakassociated with the reflected light beam 326 and the reflected lightbeam 332 (FIG. 3). Another example includes a correlation peakassociated with the reflected light beam 332 and the reflected lightbeam 336 (FIG. 3). Furthermore, additional correlation peaks may becaused by multiple reflections of light beams between the varioussurfaces of materials 304 and 306. One skilled in the art willappreciate that many correlation peaks will be displayed on the graph400, and that one skilled in the art will employ experience in using theOTG 100 (FIG. 3) to determine which correlation peaks are relevant tothe determination of distance 308. Thus, for convenience ofillustration, the correlation peaks illustrated on the graph 400 havebeen limited to correlation peaks that are necessary for explaining theoperation and functionality of the OTG 100 when measuring the distance322.

[0049] Summarizing, the OTG 100 shines the incident beam 314 onto thematerials 304 and 306. Portions of the incident beam 314 are reflectedback to the OTG 100 (reflected light beams 326, 332 and 336) and aredetected by the autocorrelator 110. Software analyzes the path lengthdifference associated with the reflected light beams 326, 332 and 336,with respect to the light reflected from reference surface 130. A personusing OTG 100 views the correlation peaks shown on the graph 340displayed on the display 126 residing on the PC 104.

[0050]FIG. 4 is a simplified graph 400 illustrating the correlationpeaks associated with the reflection of light from the surfaces ofmaterials 304 and 306, as described in detail below. For convenience ofillustrating the autocorrelation information on the graph 400, thevertical axis corresponding to the magnitude of the correlation peaks isnot numbered. One skilled in the art will realize that any appropriatevertical axis numbering system corresponding to the amplitude of thecorrelation peaks could have been employed, and that such a numberingsystem is not necessary to explain the nature of the correlation peaks.Similarly, the horizontal axis corresponding to distance has not beennumbered on the graph 400. One skilled in the art will realize that anyappropriate axis number system corresponding to distance could have beenemployed, and that such a numbering system is not necessary to explainthe nature of the relationship between the correlation peaks illustratedin the graph 400. Thus, one embodiment of the software generating thegraph 400 is configured to allow the user of the personal computer (PC)104 (FIG. 3) to alter the horizontal and the vertical axis numberingsystems so that the location of the correlation peaks of interest, andtheir relative separation corresponding to distance, can be meaningfullydiscerned and determined by the user of the PC 104 (FIG. 3).

[0051] Graph 400 illustrates the correlation peaks associated withreflected light beams 326, 332 and 336 detected by autocorrelator 110(FIG. 3). Information corresponding to the reflected light beams 326,332 and 336 (FIG. 3) is received from the autocorrelator 110 isprocessed by the PC 104 (FIG. 3). Thus, a plurality of correlation peaksare plotted on the graph 400. Correlation peak 410 is a large peak,plotted at the zero or reference point on the x-axis of the graph 400,that corresponds to the correlation of each the reflected light beamswith itself. For convenience of illustration, because the distance fromthe reference surface 130 residing in the probe head 112 is typicallymuch greater than the distances of interest associated with thematerials 304 and 306, only a portion of the distance between thecorrelation peaks 410 and 402 is illustrated. Thus, a portion of thehorizontal axis and a portion of the distance between the correlationpeaks 410 and 402 is omitted from the graph 400, as indicated by thebreak line 412.

[0052] The distances of interest shown in FIG. 3 are readily determinedfrom the position of the correlation peaks 402, 404 and 406 (FIG. 4).The separation between the correlation peak 404 and the correlation peak406 corresponds to the distance 322 (which also equals the sum ofdistances 312 and 308). Thus, the distance 322 is determined directly bymeasurement. The distance 308 of the gap 302 (FIG. 3) is easilydetermined by simply subtracting the known distance 312 from themeasured distance 322. Thus, forming the slot 310 in material 304 allowsfor direct measurement of the distance 322, and the calculation of thedistance 308, with a high degree of accuracy and reliability.

[0053] Other distances may be also determined from the position ofcorrelation peaks illustrated in FIG. 4. For example, the separationbetween the correlation peak 402 and the correlation peak 404corresponds to the distance 342 (FIG. 3). However, such information isnot necessary to determine the distance 308 of the gap 302.

[0054] As described above and illustrated in FIG. 3, the slot 310 isformed in the material 304 such that the distance 312 is preciselyknown. When the slot 310 is positioned between the probe head 112 andthe material 306, a cavity 320 is formed. Therefore, the distance 322 isaccurately and reliably measured. In various manufacturing and assemblyapplications, it is desirable to orient two materials relative to eachother such that a very precise gap between portions of the two materialsis established and/or maintained. For example, during the fabrication ofelectrical micro-circuits on substrates, two materials may be orientedwith respect to each other having a gap with a specified tolerancebetween the two substrates.

[0055] Such critical distances that are otherwise difficult orimpossible to measure with a specified degree of reliability andaccuracy with a conventional OTG can be measured by incorporating theabove described slot, or the slots of the various alternativeembodiments described herein. If the top substrate is not transparent, asmall area of the substrate is specially fabricated with a transparentmaterial, and a slot formed thereon, such that the measurementsdescribed herein are made.

[0056] The measurements taken as described herein can be used toinitially position portions of two materials relative to each other tocreate a specified gap distance. Also, measurements taken as describedherein can be used to reposition materials to maintain a specified gapdistance and/or a gap distance that is less than or equal to a specifiedtolerance. Here, the determined gap distance is compared with thespecified tolerance. At least one of the materials is repositioned todecrease the error distance. Furthermore, measurements taken asdescribed herein can be used for quality control of fabricated unitshaving a gap distance. One skilled on the art will appreciate that thereare unlimited uses of the measurements taken as described herein. Theabove described exemplary uses of the measurements taken as describedherein are illustrative of some of the possible applications in whichsmall distance measurements must be taken to measure the gap distancebetween portions of two materials. Therefore, any such applicationwherein the gap distance between portions of two materials is measuredusing the measurement method and system as described herein is intendedto be disclosed herein and be protected by the accompanying claims.

[0057] As described above and illustrated in FIG. 3, the slot 310 isformed in the material 304 to precisely determine the distance 312. Whenthe slot 310 is positioned between the probe head 112 and the material306, a cavity 320 is formed. Thus, the distance 322 is accurately andreliably measured. However, the above described embodiment requires thatthe slot 310 be formed into the material 304. In some situations, it isdesirable that the structural integrity of the material 304 not benegatively impacted. Sufficient material must remain between thesurfaces 324 and 330 to maintain the structural integrity of thematerial 304. That is, the remaining material must be thick enough, asindicated by the distance 342, for sufficient structural strength to thematerial 304. In such a situation where the formation of the slot 310negatively impacts the structural integrity of the material 304, otheralternative embodiments, described below, are desirable to form ameasurable cavity.

[0058] Furthermore, the separation of the correlation peaks 404 and 402should be sufficiently great so that the user of the OTG 100 is able toaccurately and reliably discern the position of the correlation peak 404since the separation of the correlation peaks 404 and 402 corresponds tothe distance 342 between surfaces 324 and 330 (FIG. 3). That is, if thedistance 342 is too small, correlation peak 402 may overlap, partiallyor entirely, the correlation peak 404 such that the position ofcorrelation peak 404 is not readily discernable. In such a situation,the distance 322 of the cavity 320 might not be accurately or reliablymeasured.

[0059] An alternative embodiment of the invention employs a holedisposed part way through one of the measured materials and having asuitable diameter. Thus, the amount of removed material necessary tocreate a suitable measurable cavity is minimized. By minimizing theamount of removed material, an embodiment employing a hole minimizes thenegative impact to the structural integrity of the material byminimizing the amount of removed material and/or by minimizing the sizeof the structurally weakened surface area. This embodiment requires thatthe diameter of the hole be sufficiently large for the transmission ofthe incident beam and the reflected beams, so that the reflected beamsare detectable with a sufficient degree of reliability and accuracy. Forconvenience of disclosing the invention, any suitable hole used to forma measurable cavity is defined as a slot.

[0060] Another embodiment of the invention employs a suitable recess,such as a groove, channel, slot or other suitable elongated deformationhaving a limited length. By limiting the length of the groove, channel,slot or other suitable elongated deformation, the negative impact to thestructural integrity of the material is mitigated by minimizing theamount of removed material and/or by minimizing the size of thestructurally weakened surface area. This embodiment requires that thelength of the groove, channel, slot or other suitable elongateddeformation be sufficiently long enough for the transmission of theincident beam and the reflected beams. Thus, the reflected beams aredetectable with a sufficient degree of reliability and accuracy. Forconvenience of disclosing the invention, any suitable groove, channel,slot or other suitable elongated deformation having a limited lengthused to form a measurable cavity is defined as a slot.

[0061] Other embodiments of the invention employ any suitabledeformation in the measured materials so that a measurable cavity isformed. Such a deformation may be formed in any suitable shape anddimension so long as a suitable cavity is formed in the material. Forconvenience of disclosing the invention, any suitable deformation usedto form a measurable cavity is defined as a slot.

[0062]FIG. 5 is a block diagram illustrating the OTG (not shown)measuring a gap 502 between the two materials 504 and 506 having a slot508 formed in the material 506. When the slot 508 is positionedunderneath the probe head 112 and the material 504, a cavity 510 isformed. The distance 512 associated with cavity 510 is measurable by theOTG (not shown) in which the probe head 112 resides.

[0063] Beam 514 is incident upon the top surface 516 of the material504. A light beam 518 is reflected back up into the probe head 112.Similarly, the unreflected portion of the incident beam 514 travelsthrough the transparent material 504 and is incident upon the bottomsurface 520 of the material 504. Thus, a light beam 522 is reflectedback up through the material 504 and into the probe head 112.

[0064] Material 504 is a transparent material, or a partiallytransparent material, that allows the incident beam 514 and anyreflected beams to travel through the material 504. For convenience ofillustration, only a portion of the material 504 is shown, as indicatedby the cut-away lines 524. The material 506, positioned below material504, may be transparent, partially transparent or opaque. Forconvenience of illustration, only a portion of the material 506 isshown, as indicated by the cut-away lines 526.

[0065] With this embodiment, the slot 508 is formed in the material 506.When the incident beam 514 travels through the material 504 and isincident upon the surface 528, a reflected light beam 530 is reflectedback up through the material 504 and into the probe head 112.

[0066] An autocorrelator (not shown) detects the return light so thatcorrelation peaks are determined and displayed on a graph (not shown).The correlation peaks resulting from measurements of this embodiment aresubstantially similar to the above-described embodiment wherein the slot310 was formed in the material 304 (FIG. 3). Thus, the distance 512associated with the cavity 510 is reliably and accurately measured bythe OTG because the distance 512 is greater than the minimum distancethat the OTG can reliably and accurately measure. Since the distance532, associated with the slot 508, is precisely known, the gap distance534 is determined by simply subtracting the known distance 532 from themeasured distance 512. Distance 534 corresponds to the width of the gap502. Thus, the size of the gap 502 is easily determined.

[0067] This alternative embodiment employing a slot 508 formed in thematerial 506 is particularly advantageous when it is inconvenient toform a slot into the material 504. For example, the material 504 may notbe suitable for easily forming a slot, may be too thin to form a slothaving a sufficient depth to form a measurable cavity, or may havecomponents residing in the material 504 such that a slot cannot beformed without negatively impacting the functionality of the material504.

[0068]FIG. 6 is a block diagram illustrating the OTG of FIG. 1 measuringa gap 602 between the two materials 604 and 606. This embodiment of theinvention employs a slot 608 in material 604 and a slot 610 in material606. When the slots 608 and 610 are positioned underneath the probe head112, a cavity 612 is formed. The distance 614 associated with the cavity612 is measurable by the OTG (not shown) in which the probe head 112resides.

[0069] A beam 616 is incident upon the top surface 618 of the material604. A light beam 620 is reflected back up into the probe head 112.Similarly, the unreflected portion of the incident beam 616 travelsthrough the transparent material 604 and is incident upon the surface622 of the slot 608 residing in the material 604. A light beam 624 isreflected back up through the material 604 and into the probe head 112.

[0070] Material 604 is a transparent material, or a partiallytransparent material, that allows a portion of the incident beam 616 andany reflected beams to travel through the material 604. For convenienceof illustration, only a portion of the material 604 is shown, asindicated by the cut-away lines 626. Material 606 may be transparent,partially transparent or opaque. For convenience of illustration, only aportion of the material 606 is shown, as indicated by the cut-away lines628.

[0071] With this embodiment, the slot 608 is formed in the material 604and the slot 610 is formed in the material 606. When the incident beam616 travels through the material 604 and is incident upon the surface630 formed by the slot 610, a light beam 632 is reflected back upthrough the material 604 and into probe head 112.

[0072] An autocorrelator (not shown) detects the return light such thatcorrelation peaks are determined and displayed on a graph (not shown).The correlation peaks resulting from measurements of this embodiment aresubstantially similar to the above-described embodiment wherein a singleslot 310 was formed in the material 304 (FIG. 3). Thus, the distance 614associated with the cavity 612 is reliably and accurately measured bythe OTG because the distance 614 is greater than the minimum distancethat the OTG can reliably and accurately measure. The distance 634,associated with the slot 608, is precisely known. Similarly, thedistance 636, associated with the slot 610, is precisely known.Therefore, the gap distance 638 is determined by simply subtracting theknown distances 634 and 636 from the measured distance 614. The distance638 corresponds to the width of the gap 602. Thus, the size of the gap602 is easily determined.

[0073] The alternative embodiment of the invention above employing aslot 608 formed in the material 604, and a slot 610 formed in thematerial 606, is particularly advantageous when it is inconvenient toform a slot into either materials 604 or 606 alone. For example, thematerial 604 may not be suitable for easily forming a single large slot,may be too thin to form a single large slot having a sufficient depth toform a measurable cavity, or may have components residing in thematerial 604 such that a single large slot cannot be formed withoutnegatively impacting the functionality of the material 604. Similarly,the material 606 may not be suitable for easily forming a single largeslot, may be too thin to form a single large slot having a sufficientdepth to form a measurable cavity, or may have components residing inthe material 606 such that a single large slot cannot be formed withoutnegatively impacting the functionality of the material 606. Thus,forming a shallower slot in each of the materials 604 and 606 creates acavity 612 that can be reliably and accurately measured by the OTG.

[0074]FIG. 7 is a block diagram illustrating the OTG 100 (FIG. 1)measuring a gap 702 between the two materials 704 and 706. Material 704has a slot 708. When the slot 708 is positioned underneath the probehead 112, a cavity 710 is formed. The distance 712 associated with theslot 708 is measurable by the OTG (not shown) in which the probe head112 resides. That is, material 704 is sufficiently thick to form a slot708 having a depth (corresponding to distance 712) that is at leastequal to the smallest distance that the OTG can reliably and accuratelymeasure. For example, if the smallest distance that the OTG can measureis 10 Em, the distance 712 is at least 10 μm. Preferably, the distance712 is greater than 10 μm.

[0075] When an incident beam (not shown) is incident upon the topsurface 714 of the material 704, the unreflected portion of the incidentbeam travels through the transparent material 704 and is incident uponthe surface 716. Thus, a light beam 718 is reflected back up through thematerial 704 and into the probe head 112. A portion of the incident beamcontinues to travel through the transparent material 704 and the cavity710, and is incident upon the surface 720 such that a light beam 722 isreflected back up through the cavity 710 and the material 704, and intothe probe head 112. Furthermore, another portion of the incident beamtravels through the full thickness of the material 704 (not through theslot 708) and through the gap 702, and is incident upon the surface 720.A light beam 724 is reflected back up through the gap 702 and the fullthickness of the material 704, and into probe head 112. Material 704 isa transparent material, or a partially transparent material, that allowsthe incident beam and any reflected beams to travel through the material704. For convenience of illustration, only a portion of the material 704is shown, as indicated by the cut-away lines 726. Material 706 may betransparent, partially transparent or opaque. For convenience ofillustration, only a portion of the material 706 is shown, as indicatedby the cut-away lines 728.

[0076] An autocorrelator (not shown) detects the return light (reflectedbeams 718, 722 and 724). Thus, correlation peaks are determined anddisplayed on a graph (not shown). The correlation peaks resulting frommeasurements of this embodiment are substantially similar to theabove-described embodiment wherein a single slot 310 was formed in thematerial 304 (FIG. 3). However, correlation peaks associated with beam724 will be displayed on the graph that will be used to determine themeasured distances. Furthermore, the distance 712 associated with theslot 708 is reliably and accurately measured by the OTG because thedistance 712 is greater than the minimum distance that the OTG canreliably and accurately measure.

[0077] The distance 730 associated with the slot 708 and the gap 702 isreliably and accurately measured by the OTG because the distance 730 isgreater than the minimum distance that the OTG can reliably andaccurately measure. The distance 732 is determined by simply subtractingthe calculated distance 712 (of the slot 708) from the measured distance730 (of the cavity 710). The distance 732 corresponds to the width ofthe gap 702. Thus, the width of the gap 702 is easily determined.

[0078] Furthermore, the distance 732 may be determined if the refractiveindex n of material 704 is known. The distance traveled by the reflectedbeams 722 and 724 are equal, assuming that the surface 720 is flat andthat the reference surface 130 is aligned parallel to the surface 720. Atime delay is induced in the reflected beam 724, with respect to thereflected beam 722, because of the refractive index n of material 704.That is, since the reflected beam 722 (and the incident beam) travelsthe distance 712 through air, and the reflected beam 724 (and theincident beam) travels the distance 712 through the material 704, thereflected beam 724 is delayed compared to the reflected beam 722, by afactor that corresponds to the refractive index n of material 704.Knowing the index of refraction n, the distance 712 of the slot depthcan be easily calculated from the distances measured on the OTG betweenthe peaks from reflected beams 722 and 724. The distance 732 isdetermined by subtracting distance 712 from measured distance 730. Thus,the distance 732 is easily determined by measuring the separation of thecorrelation peaks associated with the reflected beams 718, 722 and 724,and by relating the measured separation of the correlation peaks withthe refractive index n.

[0079] The alternative embodiment above employing a slot 708 formed inthe material 704 is particularly advantageous when it is inconvenient toprecisely measure the distance 712 of the slot 708, or if the slot 708having a precisely known distance 712 is difficult or impossible to formin the measured materials. The user measuring the gap 702 need onlyshine the incident beam onto the material 704 and 706 so that thereflected beams 718, 722 and 724 are detected by the OTG. That is, aslot having any depth (at least equal to the minimum distance that theOTG can reliably and accurately measure) is formed in the material 704in a convenient manner, and all necessary distance measurements aretaken to reliably and accurately determine the distance 732 associatedwith the gap 702.

[0080] Alternatively, in the event that material 704 is not sufficientlythick enough for the above described gap 708, a similar gap (having adepth at least equal to the minimum distance that the OTG can reliablyand accurately measure) may be formed in the material 706. Thus,measurements are taken with the OTG to reliably and accurately determinethe distance 732 associated with the gap 702.

[0081] For convenience of describing the functionality and operation ofthe OTG 100 (FIG. 1), the OTG 100 was described as employinglow-coherence light generated by the low-coherence light source 106.Alternative embodiments of the present invention employ othertransmittable, low-coherence energy spectrums. Waves associated with theselected spectrum are shined upon the surfaces of materials having slotsor the like as described above. The reflected waves are then correlatedto accurately and reliably measure a gap between two materials. Forexample, a wave residing in the infra red portion of the energy spectrumcould be selected.

[0082] Another embodiment of the present invention employs slotsdisposed in the materials in accordance with the above describedembodiments. However, measurements of the distances are taken with asplit-beam OTG constructed in accordance the copending and commonlyassigned U.S. patent application Ser. No. 09/929,767, filed on Aug. 14,2001, and entitled OPTICAL MEASUREMENT SYSTEM AND METHOD FOR DETERMININGHEIGHT DIFFERENTIAL BETWEEN TWO SURFACES, which is entirely incorporatedherein by reference.

[0083] It should be emphasized that the above-described embodiments ofthe present invention, particularly, any “preferred” embodiments, aremerely possible examples of implementations, merely set forth for aclear understanding of the principles of the invention. Many variationsand modifications may be made to the above-described embodiment(s) ofthe invention without departing substantially from the principles of theinvention. All such modifications and variations are intended to beincluded herein within the scope of this disclosure and the presentinvention and protected by the following claims.

Now, therefore, the following is claimed:
 1. A system for measuring adistance between materials comprising: a first material having asurface; a second material having a surface, the surface of the secondmaterial being separated from the surface of the first material by agap; and a slot disposed in the first material such that a slot surfaceis formed on the first material, the distance between the surface of thefirst material and the slot surface being a precisely known distance,such that when a cavity distance is measured between the surface of thesecond material and the slot surface, a gap distance is determined bysubtracting the precisely known distance from the cavity distance. 2.The system of claim 1, further comprising a second slot disposed in thesecond material such that a second slot surface is formed on the secondmaterial, the distance between the surface of the second material andthe second slot surface being a precisely known second distance, suchthat when a cavity distance is measured between the slot surface and thesecond slot surface, the gap distance is determined by subtracting theprecisely known distance and the precisely known second distance fromthe cavity distance.
 3. A method for measuring distance between twomaterials, the method comprising the steps of: measuring distancebetween a slot surface formed by a slot in a first material and asurface on a second material, the first material having a surface suchthat the distance between the slot surface and the surface of the firstmaterial is a precisely known distance; and subtracting from themeasured distance the precisely known distance to determine the distancebetween the first material and the second material.
 4. The method ofclaim 3, further comprising the step of measuring the precisely knowndistance between the slot surface and the surface of the first material,the step of measuring the precisely known distance completed before thestep of measuring distance between the slot surface formed by the slotin the first material and the surface on the second material.
 5. Themethod of claim 3, further comprising the step of measuring theprecisely known distance between the slot surface and the surface of thefirst material, the step of measuring the precisely known distanceconcurrently with the step of measuring distance between the slotsurface formed by the slot in the first material and the surface on thesecond material.
 6. The method of claim 3, further comprising the stepsof: transmitting a light through the first material and onto the surfaceof the second material; and detecting reflected light from the slotsurface and the surface of the second material such that the measureddistance is determined.
 7. The method of claim 3, further comprising thestep of forming the slot in the first material.
 8. The method of claim3, further comprising the steps of: measuring a distance between asecond slot surface and the slot surface of the first material, thesecond slot surface formed by a second slot in the second material suchthat the distance between the second slot surface and the surface of thesecond material is a second precisely known distance; and subtractingfrom the measured distance the precisely known distance and theprecisely known second distance to determine the distance between thefirst material and the second material.
 9. The method of claim 8,further comprising the step of forming the second slot in the secondmaterial.
 10. The method of claim 8, further comprising the steps of:transmitting a light through the first material and onto the second slotsurface; and detecting reflected light from the slot surface and thesecond slot surface such that the measured distance is determined. 11.The method of claim 10, further comprising the steps of: forming acavity by aligning the slot in the first material with the second slotsuch that the cavity is formed by a gap between the first material andthe second material and by the alignment of the slot in the firstmaterial with the second slot; transmitting light through the cavity tomeasure a cavity distance; and determining a gap distance by subtractingfrom the cavity distance the precisely known distance and the preciselyknown second distance.
 12. The method of claim 3, further comprising thestep of: comparing the distance between the first material and thesecond material with a predefined reference distance; and determining anerror distance corresponding to the compared distances.
 13. The methodof claim 12, further comprising the step of adjusting the position ofthe first material such that the error distance is decreased to aspecified tolerance.
 14. The method of claim 12, further comprising thestep of adjusting the position of the second material such that theerror distance is decreased to a specified tolerance.
 15. A system formeasuring distance between two materials, comprising: means formeasuring distance between a slot surface formed by a slot in a firstmaterial and a surface on a second material, the first material having asurface such that the distance between the slot surface and the surfaceof the first material is a precisely known distance; and means forsubtracting from the measured distance the precisely known distance todetermine the distance between the first material and the secondmaterial.
 16. The system of claim 15, further comprising: means fortransmitting a light through the first material and onto the surface ofthe second material; and means for detecting reflected light from theslot surface and the surface of the second material such that themeasured distance is determined.
 17. The system of claim 15, furthercomprising: means for measuring a distance between a second slot surfaceand the slot surface of the first material, the second slot surfaceformed by a second slot in the second material such that the distancebetween the second slot surface and the surface of the second materialis a second precisely known distance; and means for subtracting from themeasured distance the precisely known distance and the precisely knownsecond distance to determine the distance between the first material andthe second material.
 18. The system of claim 17, further comprising:means for transmitting a light through the first material and onto thesecond slot surface; and means for detecting reflected light from theslot surface and the second slot surface such that the measured distanceis determined.
 19. The system of claim 18, further comprising: means forforming a cavity by aligning the slot in the first material with thesecond slot such that the cavity is formed by a gap between the firstmaterial and the second material and by the alignment of the slot in thefirst material with the second slot; means for transmitting lightthrough the cavity to measure a cavity distance; and means fordetermining a gap distance by subtracting from the cavity distance theprecisely known distance and the precisely known second distance.